[Fargate] [request]: Capacity Reservations for Fargate

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**Tell us about your request**
Add the ability to create Capacity Reservations for Fargate in order to guarantee capacity in a high load situation (or describe why this wouldn't ever be the case)

**Which service(s) is this request for?**
Fargate for ECS and EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Ensure that capacity is always available for Fargate to be provisioned within an environment.

EC2 uses Capacity Reservations in order to guarantee capacity be provided when the service is experiencing high-load and cannot service usual requests for compute capacity (for example, during an AZ outage, capacity typically spikes). I would like the same for Fargate to guarantee (pre-provision) capacity can be provided.

I can also see this might come with a side benefit of ensuring warm compute being always available (up to your capacity limits) for quicker provision times.

**Are you currently working around this issue?**
Hopes and prayers.

**Additional context**
Relates to #109
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capacity-reservations.html

**Attachments**
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